1. Field of the Invention
The invention relates generally to a sensorless control unit for a brushless direct-current (DC) motor and, more specifically to a sensorless control unit for a brushless DC motor that is suitable for driving a pump that sucks in and discharges oil.
2. Discussion of Background
Hydraulic pressure is supplied to a transmission of a vehicle by a hydraulic pump. In a vehicle that carries out so-called idling stop, which is a technique of stopping an engine when the vehicle stops, in view of, for example, energy-saving, an electric hydraulic pump is used to reliably supply hydraulic pressure to the transmission even during idling stop.
A brushless DC motor is used as an electric motor for driving a pump, which is mounted in a vehicle. In addition, so-called sensorless control for driving the motor without the use of a rotational position detection sensor is executed.
In order to execute the sensorless control over the brushless DC motor, it is necessary to estimate the rotational position of a rotor and generate an estimated rotational position signal that corresponds to a rotational position signal from a rotational position detection sensor. The estimated rotational position signal is usually generated with the use of three-phase induced voltages of the motor. However, when the rotor is not rotating or is rotating at a low speed during start-up of the motor, induced voltages are zero or low. Therefore, it is not possible to generate an estimated rotational position signal. Accordingly, as described in, for example, Japanese Patent Application Publication No. 2005-278320 (JP 2005-278320 A), the pattern of electric power supply to the three phases is forcibly changed at regular intervals to generate a revolving magnetic field to thereby carry out forced commutation for forcibly rotating the rotor.
In addition, a conventional sensorless control unit for a brushless DC motor executes a sensorless control mode using the rated current value of a motor as the maximum current value.
When the above-described conventional sensorless control unit for a brushless DC motor is applied to a brushless DC motor for driving a hydraulic pump for a transmission of a vehicle, the following problem may occur. When the hydraulic load is high (the oil temperature is low), it takes a long time to bring the hydraulic pressure to a required hydraulic pressure. This makes it difficult to start up the motor promptly.